Spectral sensitization is an extremely important and essential technique in the preparation of a high sensitivity stable silver halide photographic materials. Heretofore, variousspectral sensitizers have been developed. At the same time, technical developments have been made in the usage of these spectral sensitizers, e.g., how they can be used for supersensitization, how they can be added to materials.
It has been known that spectral sensitizing dyes for spectral sensitization include spectral sensitizers such as cyanine dyes, melocyanine dyes and rhodacyanine dyes used singly or in combination (e.g., for supersensitization).
In order to serve as sensitizing dyes of photographic material, they are required to provide a high spectral sensitivity as well as to meet many other requirements. For example, they must give no fog sensitization. They also must exhibit excellent properties upon exposure, e.g., latent image stability, reciprocity characteristics, temperature and humidity dependence, upon exposure. Further, when unexposed, such sensitizers must show little change in the sensitivity, gradation and fog from before to after storage. Moreover, they must not remain in the photographic material after development.
Among these requirements, the high sensitivity and high storage stability are essential. Many efforts have been made to meet these requirements. These efforts are described in JP-A-60-202436, JP-A-60-220339, JP-A-60-225147, JP-A-61-123834, JP-A-62-87953, JP-A-63-264743, JP-A-1-15534, JP-A-1-177533, JP-A-1-198743, JP-A-1-216342, JP-A-2-42 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") , JP-B-60-57583 (the term "JP-B" as used herein means an "examined Japanese patent publication"), and U.S. Pat. No. 4,618,570. However, the level of the sensitivity and storage stability attained by these efforts leaves much to be desired.
In the silver halide photographic materials, it is important to incorporate various dyes into the materials to enhance the sharpness and color separability thereof.
On the other hand, supersensitization is described in Photographic Science and Engineering, vol. 13, pp. 13-17 (1969), and vol. 18, ibid., pp. 418-430 (1974), and James, The Theory of the Photographic Process, 4th ed., page 259 (Macmillan, 1977). It is known that the selection of proper sensitizing dyes and supersensitizer can give a high sensitivity.
A dye in which the 2'-position in the propylene crosslink attached to the 2- and 4-positions on the methine chain is mono-substituted is described in British Patents 595,783, 595,784, 595,785 and 604,217, and U.S. Pat. Nos. 2,481,022 and 2,756,227. Referring to its spectral sensitizing capacity in a silver halide system, U.S. Pat. No. 2,481,022 states that the spectral sensitization of silver bromoiodide with such a dye gives a spectral sensitivity maximum of 695 to 710 nm.
Thus, all conventional polymethine dyes containing a propylene crosslinking structure attached to the 2- and 4-positions on the methine chain are spectrally sensitized in M-band (Addition method: stored at a temperature of 40.degree. C. for 20 minutes after addition of a methanol solution of such a dye).
The inventor made an extensive study to utilize J-band spectral sensitization to enhance sensitivity and storage stability. As a result, it was found that J-band spectral sensitization can be accomplished with a structure in which the 2'-position in the propylene crosslink attached to the 2- and 4-positions on the methine chain is mono-substituted. The inventor has already described such compounds in JP-A-4-146428, JP-A-4-146430, and JP-A-4-146431.
However, these conventional compounds occasionally give insufficient spectral sensitivity in certain wavelength ranges and leave much to be desired in storage stability.